Galactic Archaeology with Gaia GyuChul Myeong Institute of Astronomy University of Cambridge This dissertation is submitted for the degree of Doctor of Philosophy Pembroke College July 2019 Declaration I hereby declare that except where specific reference is made to the work of others, the contents of this dissertation are original and have not been submitted in whole or in part for consideration for any other degree or qualification at the University of Cambridge, or any other University. This dissertation is the result of my own work and includes nothing which is the outcome of work done in collaboration, except where specifically indicated. This dissertation contains less than 60,000 words including abstract, appendices, footnotes, tables and equations and has less than 150 figures. GyuChul Myeong July 2019 Abstract GyuChul Myeong: Galactic Archaeology with Gaia The halo of our Galaxy is believed to be mainly formed by the materials accreted/merged in the past, and so has “extragalactic” origin. Such formation process will leave dynamical traces imprinted in the halo, like stellar substructures, distinguishable from the in-situ halo component. Studying the present-day structure and substructures of the Milky Way halo is one of the most direct ways of understanding the formation and the evolutionary history of the Galaxy, as well as investigating the LCDM model on the galaxy scale which has not yet been tested thoroughly. It has been a challenge to obtain a sufficiently large sample of halo stars for such study due to the sparse density of the halo. The recent Gaia mission can open a new era for the study of Galactic Archaeology as it provides quality data for ∼ 1:3 billion stars across the Milky Way which had remained uncharted so far. In Chapter1, I describe a history of study on the Milky Way halo so far, and present algorithms that are developed to investigate the substructures of the halo with various aspects. Chapter2 is a morphological study of the Milky Way halo based on the chemo-dynamical information. It reveals various interesting aspects of the halo and its origin, such as the chemodynamical duality (evidence of a past major merger – the “Gaia Sausage”), traces of a past retrograde accretion (clues as to the origin of the retrograde halo component), and the resonant feature (evidence of dynamical influence of the Milky Way bar). Chapter3,4 and5 are examples of a more focused study on the halo substructures with various new methods that differ from the conventional studies. In addition to the discovery of new stellar streams, I investigate the properties of the potential progenitors (past accreted dwarf galaxies) of these substructures, and also the potential association with w Centauri. Chapter6 is a study investigating the potential extragalactic origin of the Milky Way globular clusters based on their dynamics and various other information such as age, metal- licity, horizontal branch index. It reveals a collection of globular clusters with extragalactic origin, originating from the “Gaia Sausage”. Chapter7 is a chemo-dynamical study showing the evidence for another early accretion event – the “Sequoia”. From multiple tracers in the Milky Way halo, including the stellar streams and globular clusters, I investigate the dynamical and the chemical signature of the “Sequoia” progenitor and its present-day remnants. To my loving family Acknowledgements I wish to express my most sincere thanks to my supervisors, Prof. N. Wyn Evans and Prof. Vasily Belokurov, for their continuous support and guidance throughout my study and research. Their immense knowledge and wisdom coupled with their warm encouragement have been a great help to me throughout my entire Ph. D. years. I am profoundly grateful to have been under their supervision and the past three years could not have been more fun. My sincere thanks also go to Dr. Jason Sanders, Dr. Eugene Vasiliev, Prof. Sergey Koposov, Dr. Denis Erkal, Dr. David Aguado, Dr. Nicola Amorisco and every member of the Cambridge Streams research group for sharing their knowledge and experience with me and for numerous helpful discussions. My heartfelt thanks go to Prof. Ken Freeman and Prof. Rosemary Wyse for their invalu- able supports and encouragements. I cannot express my appreciation enough for the amazing opportunities that they have created for me. A special gratitude goes out to Mr. Fadi Boustany for his warm supports throughout my study and for providing me with an internship opportunity in Monaco. I wish to thank the Boustany Foundation, especially Ms. Alison Pitt, the Isaac Newton Studentship and Cambridge Trust for supporting my study. I would like to thank the staffs at the Institute of Astronomy and Pembroke College, especially Prof. Paul Hewett and Ms. Debbie Peterson, for their precise and effective supports. I also wish to thank Ms. Margaret de Vaux for her kind and thoughtful supervisions which have been very helpful. Dr. Giuliano Iorio always has been a great colleague and a thoughtful friend. I would also like to thank my fellow students, especially James Grady, Peter McGill, Daniel Muthukrishna and Luis Welbanks for being amazing friends and for making good memories. Above all, I wish to send my deepest thanks to my family, especially to my parents, RohSun Myeong and JiEon Lim, and my sister, GyuRi Myeong, for their endless love and support. I am truly grateful for their unceasing encouragement. Table of contents List of figures xiii List of tables xxvii 1 Introduction1 1.1 Milky Way Halo and Galactic evolution . .1 1.1.1 Milky Way Globular clusters . .3 1.1.2 Halo Substructure . .4 1.1.3 Dark matter detection with Halo streams . .7 1.1.4 Phase space . .9 1.1.5 Rotation of Halo . 11 1.2 Halo stars in the Gaia era . 12 1.3 Algorithm construction . 14 1.3.1 Multi-dimensional approach . 15 1.3.2 Photometric approach . 17 1.4 Outline . 20 2 Milky Way Halo in Action Space 21 2.1 Introduction . 22 2.2 The SDSS–Gaia Catalogue . 23 2.3 Characteristics of the Halo in Action Space . 25 2.3.1 The Rich and the Poor . 25 2.3.2 The Retrograde Stars . 27 2.3.3 The Resonant Stars and the Hercules Stream . 27 2.4 Discussion . 29 3 A Halo Substructure in Gaia Data Release 1 31 3.1 Introduction . 32 3.2 Extraction of the Member Stars . 33 x Table of contents 3.3 Discussion and Conclusions . 39 4 Halo Substructure in the SDSS-Gaia Catalogue: Streams and Clumps 43 4.1 Introduction . 44 4.2 Method . 47 4.2.1 Sample . 47 4.2.2 Detection . 53 4.3 Candidates . 57 4.3.1 The Hotter Substructures: S1, S3 and S4 . 57 4.3.2 The Colder Substructures: S2, C1 and C2 . 59 4.3.3 Distribution on the Sky . 60 4.4 Interpretation . 60 4.5 Conclusions . 64 5 Discovery of New Retrograde Substructures: The Shards of w Centauri? 67 5.1 Introduction . 68 5.2 Detection of Substructures . 74 5.2.1 Method . 74 5.2.2 Algorithm Validation . 76 5.3 Substructure Forensics . 82 5.3.1 Cross-checks: Known Candidates (S1, S2, C2) . 82 5.3.2 The Retrograde Candidates . 84 5.4 Conclusions . 86 6 The Sausage Globular Clusters 89 6.1 Introduction . 90 6.2 The Globular Clusters in Action Space . 91 6.3 The Sausage Globular Clusters . 97 6.4 Discussion . 99 7 Evidence for Two Early Accretion Events That Built the Milky Way Stellar Halo 101 7.1 Introduction . 102 7.2 The Nature of FSR 1758 . 104 7.2.1 Data . 104 7.2.2 Dynamical Modelling . 106 7.2.3 Photometric modelling . 110 Table of contents xi 7.3 Tracers of the Sequoia Galaxy . 112 7.4 The Sequoia and the Sausage . 121 7.5 Conclusions . 126 8 Summary and Future Prospects 129 References 135 Appendix A Tidal tails around the outer halo globular clusters Eridanus and Palo- mar 15 169 A.1 Introduction . 170 A.2 Data Analysis . 171 A.3 Eridanus . 174 A.4 Palomar 15 . 174 A.5 Discussion . 176 List of figures 1.1 Contours of effective equipotential Feff for two point masses with a mass ratio of m=M = 1=9 in a circular orbit (see Chapter 8.3 of Binney and Tremaine, 2008, and also, Eq. 1.1). The Lagrange points are marked as L1;2;3;4;5. Two unstable Lagrange points, L1 and L2, are the main gateways for escaping stars. [Image Credit: Binney and Tremaine(2008)] . .5 1.2 Algorithm structure for the Multi-dimensional approach algorithm . 16 1.3 Algorithm structure for the Photometric approach algorithm . 19 2.1 Histograms of the stellar halo in action space (JR;Jf ), (JR;Jz) and (Jf ;Jz) split into metal-rich (left column, −1:6 < [Fe/H] < −1:1) and metal-poor (middle column, −2:9 < [Fe/H] < −1:8). The right column displays the difference, with red showing an excess of metal-rich, blue an excess of metal-poor stars. Notice (i) the metal-rich stars are tightly clustered around Jf ≈ 0 and are much more extended in JR, and (ii) the metal-poor stars have prograde rotation (hJf i > 0) and a more isotropic distribution in action space. 24 2.2 The distribution of halo stars in energy-action space or (Jf ;E) and (JR;E) space, split according to six metallicity bins from −2:9 < [Fe/H] < −1:3. As we move from the most metal-rich to the most metal-poor, notice that (i) the distribution in radial action becomes more compact as the tail melts away by [Fe/H] ≈ −2:0, (ii) the diamond-like shape of the contours in (Jf ;E) changes gradually into an upturned bell-like shape, and (iii) the high energy, retrograde stars (marked by rectangular boxes in the left panels) and the high energy, eccentric stars (boxes in the right panels) gradually disappear by [Fe/H] ≈ −1:9, (iv) there is a distinct prograde component at Jf ≈ 1100, −1 2 −2 JR ≈ 150 km s kpc, E ≈ −1:6 km s (marked by ellipses), which is present at all metallicities.